I have installed Mizu softphone and set it up with my voip provider, Localphone. The softphone registered but to my
surprise there is a MizuVoIPServer which registers as well. What is the role of this server?
Thank you.

The reason is to enable push notification support.Push notifications are good to be able to accept incoming calls/chat while the app is not running or your device is on sleep.Most servers doesn't have built-in push notification support and the softphone will send a second register via our push gateways.You can disable this behaviour by disabling push notifications from the settings. In that case the sofphone can still accept incoming calls by running its service in the background.